The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Competition in the automotive industry has increased the need to develop technology for superior engines. Electronic monitoring and microsecond control systems provide a level of sophistication and performance that has not previously been available in automotive engines. Accurate input to these control systems has become an important priority. Of particular relevance is the precise control of the air-fuel ratio. Electronic port fuel injectors have nearly become standard on today's modern engine. These devices have achieved a very high degree of reliability and accurate fuel delivery control. Combustion air control is equally important, and the measurement of this air flow is done with a mass air flow sensor or a manifold absolute pressure sensor.
FIG. 1 shows an example of the induction air velocity downstream of the air cleaner in the induction system of a production four cylinder internal combustion engine. Shown are regions of negative and positive flows. Although the mass air flow sensor unit has the advantage of measuring the mass air flow rate directly, special problems arise in the measurement of this combustion air in that traditionally only part of the intake stream is sampled and the total mass flow is estimated from this bypass fraction. FIG. 1 shows the flows in the bypass tube and main bore of traditional mass air flow sensors. Shown are comparisons of the bypass flows with total mass flows in the main bore at various engine speeds with different throttle positions under motoring and firing conditions. FIG. 1 additionally shows a typical example of air flow in the induction system of an engine under normal operating conditions. Complicated induction system flows make an accurate correlation of bypass flow to total flow a difficult problem, particularly in operating regions which exhibit flow reversals. FIGS. 2A and 2B show an example of a prior art typical mass air flow sensor assembly having a single hotwire sensor which measures the mass flow of air passing through the air intake by measuring changes in resistance caused by heat loss.
The graph shows mass air flow sensor signals measured along with crank angles over a range of throttle positions. The mass air flow sensor signals were measured to be compared with velocities measured by laser doppler velocimetry in the main base and the bypass tube of the mass air flow sensor assembly. This comparison can be used to determine the differences of the sensitivity and response time between the laser doppler velocimetry system and the mass air flow hotwire sensor.
FIGS. 3A and 3B show total flow rates calculated based on the average velocity over 720 crank angle degrees across unit sectional area. Here, flow rate (+) means a flow rate calculation based only on the positive velocities, while flow rate (abs) is calculated based on the average velocities of absolute values of measured velocity. The flow rate without a parenthetical notation stands for net the flow rate. For lower than 50% of throttle position, these three flow rates are identical because no backflow exists in those throttle positions. For higher than 50% of throttle position, the backflow affects the flow rate in main bore. The net flow rate was decreased with increasing the throttle position at throttle position higher than 50%.
A similar trend was found when the measured flow just upstream of the bypass was used to calculate total flow rate. Since the backflow at the entrance to the bypass is small, the effect of backflow is small on the flow rate in the bypass. The net flow rate through the bypass increases when the throttle position increases with the flow rate (abs) and the flow rate (+). This is different from the flow rate in the main bore.
A mass air flow sensor which can provide the current net flow rate into an engine assembly will promote enhanced control of air-fuel ratios in the combustion chamber of an internal combustion engine. This has particular significance for: Premix gasoline engines operating above mid throttle position; diesel engines especially during transients; stratified charge spark ignition engines; in all engines which have variable cam timing the calibration effort can be substantially reduced by implementation of an accurate mass air flow sensor.